1. Field of the Invention
The present invention relates to an organic composite insulator used for a bushing, cable head, etc. and a method of producing the same.
2. Description of the Related Art
FIG. 1 to FIG. 3 are sectional views of organic composite insulation tubes as the related art.
Organic composite insulator tubes shown in FIG. 1 to FIG. 3, are insulator tubes comprised of an FRP tube or other organic material tube 1, voltage application side and ground side end fittings 2, 2xe2x80x2 of iron, aluminum, copper, or an alloy of them fastened to the two ends, and an insulator sheath 3 of an organic polymer material such as silicone rubber, EPDM, or EVA molded at the outside of the organic material tube between the two end fittings 2, 2xe2x80x2 and parts of the two end fittings 2, 2xe2x80x2. The inside of the organic composite tube is hollow.
The shape is, as shown in FIG. 1 and FIG. 3, that of a straight type of an inside diameter of the pipe portion constant between the top and bottom ends such as an FRP tube or, as shown in FIG. 2, a tapered type of an inside diameter of a bottom larger than an inside diameter of the top.
In both cases, the insulator sheath 3 is comprised of bodies 3-1 and watersheds (projections) 3-2 projected from the bodies. There are cases where it is comprised of watersheds of one type of outside diameter as shown in FIG. 1 and FIG. 2 and cases of large diameter watersheds 3-2xe2x80x2 and small diameter watersheds 3-2xe2x80x3 as shown in FIG. 3. In the straight type, however, the outside diameter of the watersheds and the outside diameter of the body are formed the same from the top to the bottom.
Therefore, the mold 11 used when molding the organic polymer material on the outside of the organic material tube 1 or the outside of the organic material tube 1 and the outside of parts of the end fittings 2, 2xe2x80x2, as shown in FIG. 4, is formed by one or more unit mold portions 12 for forming a uniform body a and watersheds b except for the portions corresponding to the end metal portions 13a and 13b. 
In general, in a bushing or cable head, as shown in FIG. 5 and FIG. 6, the equipotential lines 7 concentrate near the tip 6a of the top of an inside electrode 6 for easing the electric field placed between an internal conductor 4 at the ground side end metal fitting 2xe2x80x2 and the insulator inside wall 5. The surface electric field of the sheath portion 8 corresponding to the vicinity of the tip 6a of the top of the inside electrode 6 where the equipotential lines 7 cut closest to the surface of the insulator sheath 3 becomes strong. A phenomenon occurs where the air insulation is broken first at this part, then an overall external flashover occurs.
Therefore, if the outside diameter S of the body of the sheath and outside diameters T of the watersheds of this sheath portion 8 are small, the external flashover occurs at a relatively low power voltage, a corona occurs, or the required specifications are not satisfied.
As a means for preventing this phenomenon and weakening the surface electric field of the insulator sheath in the vicinity of the tip 6a of the inside electrode 6 to obtain the required flashover characteristic or corona characteristic, normally an insulator having a sufficiently large inside diameter and outside diameter is selected.
An object of the present invention is to provide an organic composite insulator and method of production thereof, giving an external flashover characteristic similar to a large inside diameter insulator even if the inside diameter of the insulator is made smaller, enabling a reduction in the cost, and enabling a reduction in size of the insulator.
According to the present invention, there is provided an organic composite insulator comprising: an organic material insulator tube having a hollow portion into which an inside electrode for easing an electric field is inserted; an insulator sheath, a length thereof being shorter than that of the organic material insulator tube, provided on the outer circumference of the organic material insulator tube and having a plurality of body portions and watershed portions alternatively and continuously formed; a first end conductive fitting member fixedly provided on a first end of outer circumference on which the insulator sheath is not provided and on a part of a first end of the insulator sheath, the first end conductive fitting member being applied a voltage; and a second end conductive fitting member fixedly provided on a second end outer circumference on which the insulator sheath is not provided and on a part of a second end of the insulator sheath, the second end conductive fitting member being grounded; the insulator sheath being made of an organic polymer material and having a first insulator sheath, a second insulator sheath and a third insulator sheath continuously aligned from the first end conductive fitting member to the second end conductive fitting member and integraledly formed as a unit, the second insulator sheath being positioned in a vicinity of a top tip of the inside electrode inserted into the hollow portion of the organic material insulator tube from the second end conductive member, and outer diameters S-2 and T-2 of the respective body portion and watershed portion in the second insulator sheath being greater than outer diameters S-1 and T-1 of the respective body portion and watershed portions in the first and/or third insulator sheath.
According to the present invention, there is also provided a method of producing the organic composite insulator, in the method, when the insulator sheath is formed by molding, an assembled mold comprising a plurality of unit molds separated in each unit mold, the unit mold for forming the second insulator sheath having inner diameters for forming the body portions and the watershed portions of the second insulator sheath greater than these of the first and third insulator sheaths.